1. Field of the Invention
The present invention relates to a method of calibration of a magnetic resonance imaging (MRI) system and a motion compensation circuit, in particular for compensating for the effects of motion of an image subject during MRI scanning using phase encoding in a magnetic resonance imaging system.
2. Description of the Prior Art
MRI scanners use a combination of a strong constant magnetic field (B0) from a superconducting magnet which is modified by gradient fields generated by gradient coils, together with a rotating magnetic field (B1) from a radio frequency (RF) antenna to excite nuclear magnetic resonances in the body that generate short term RF signals that are received to build up a tomographic image.
All current-generation MRI scanners employ arrays of local coils mounted in close proximity to the scanned patient to receive the RF with maximum possible signal to noise ratio (SNR). The local coils that receive signals from the back of the patient are mounted in the patient table. Local coils that receive signals from the front of the patient are arranged into ‘mats’ that are carefully placed over the part of the patient for imaging. Associated with each mat is a flexible cable typically containing one co-axial line for each local coil. The cables interact with the B1 field and with the signals generated from the patient so ‘traps’ (high impedance sections) must be included at regular (typically λ/8) intervals. These add cost and inconvenience to the structure. In use, the requirement to connect the cables and sterilize them between scanning one patient and the next leads to down-time between scans. A wireless implementation allows elimination of these cables. However, in a wireless implementation, there may be errors introduced into the received signals due to motion of the patient.